The concretions probably formed when minerals precipitated out of water.

These tiny spheres were found in the rock outcrops and soil in Eagle Crater, where Opportunity touched down.

[Lion Stone] is different in subtle ways from what we saw at Eagle Crater; a little different in mineralogy, a little different in color

Professor Steve Squyres, Cornell University

Layering is characteristic of sedimentary rocks, which can be formed by water or wind. Testing showed the layered rocks at Eagle Crater were deposited in water.

Only further investigation will determine which was responsible for layering seen in rocks at the 130m-wide Endurance Crater.

But the presence of sulphur in Lion Stone is a promising sign to mission scientists that the rocks Opportunity is now examining were deposited in water.

On Earth, rocks with as much salt as these Mars rocks either have formed in water or, after formation, have been highly altered by long exposure to water.

Subtle differences

"However, [Lion Stone] is different in subtle ways from what we saw at Eagle Crater; a little different in mineralogy, a little different in colour," said Professor Steve Squyres, principal investigator for the Mars Exploration Rovers mission.

"It may give us the first hint of what the environment was like before the conditions that produced the Eagle Crater rocks."

Opportunity will spend a few weeks driving around Endurance in an anti-clockwise direction, examining the rocks along its rim. This will give Nasa scientists time to decide whether to send the robotic explorer into the depression.

Mini-Tes data is revealing the minerals in rocks at Endurance Crater

They need weigh up the scientific benefits of steering Opportunity down the crater slope against the real possibility that it may not be able to get out again.

This decision will partly be based on data obtained from the Mini-Thermal Emission Spectrometer (Mini-Tes), which the scientists are currently using to map the mineral composition of the rocks in Endurance.

"Most exciting is the basalt signature in the layered cliffs," said Professor Phil Christensen, of Arizona State University, Tempe, US, and lead scientist for the Mini-Tes.

"Basalt is volcanic in origin, but the thinness of the layers visible in the cliffs suggests they were emplaced some way other than as flows of lava.

"Our working hypothesis is that volcanically erupted rock was broken down into particles that were then transported and redeposited by wind or by liquid water."

Endurance Crater contains many exposed slabs of layered rocks that might provide information about a long period of environmental history on Mars.

Some rocks at Endurance are more than 10 times thicker than the bedrock exposure at Eagle Crater. But some rock stacks seen from orbit at other locations in Mars' Meridiani Planum region are up to 200m thick (650ft).